Illumination systems that utilize a wavelength conversion material such as phosphor to produce light of specific range of wavelengths (e.g. red, green and blue wavelengths) have advantages over illumination systems that produce these specific wavelengths directly and without using a wavelength conversion material. These advantages include better color stability, color uniformity and repeatability. In case of lasers, wavelength conversion can provide a low-cost way for producing visible light (e.g. green) when compared to frequency doubling methods. However, light coupling efficiency suffers significantly in this case due to brightness loss (i.e. etendue of converted light is much higher than that of the light source)
The prior art describes various wavelength conversion based illumination systems. For example, in U.S. patent application Ser. No. 11/702,598 (Pub. No.: US20070189352), Nagahama et al. describes a light emitting device 100 utilizing a wavelength conversion layer 30, as illustrated in FIG. 1A. The light emitting device 100 consists of a light source 10, a light guide 20, a light guide end member 47, an optional reflective film 80, a wavelength conversion member 30, a reflection member 60, and a shielding member 70. The light guide 20 transfers the light emitted from the light source 10, and guides the light to the wavelength conversion element 30. Some of this light is absorbed by element 30 and emitted at a converted wavelength. Reflective film 80 enhances the efficiency by reflecting excitation (source) light that was not absorbed back toward wavelength conversion element 30 and by also reflecting converted light toward the emission side of light emitting device 100. Reflection member 60 reflects at least part of the excitation light back toward the wavelength conversion member 30 in order to increase the light emitting efficiency. The shielding member 70 blocks the excitation light and transmits a light of a specific wavelength. In light emitting device 100, portions of source and converted light beams exit light emitting device 100 through the edges of wavelength conversion member 30, reflection member 60, shielding member 70 and reflective film 80, thus, resulting in light losses and lower optical efficiency. In addition, the reflectivity of reflective film 80 can be enhanced further, thus, reducing optical losses. Therefore, there is a need for systems that can reduce or eliminate light losses and enhance overall efficiency.
In U.S. Pat. No. 7,040,774, Beeson et al. proposes illumination system 200. As shown in FIG. 1B, illumination system 200 is comprised of a light emitting diode (LED) 116, a wavelength conversion layer 124 (e.g. phosphor), a light-recycling envelope 112 made from a reflective material (or having a reflective coating applied to its internal surfaces), an optional light guide 126, an optional optical element 125 (e.g. reflective polarizer or dichroic mirror) and a light output aperture 114. The LED 116 has a light emitting layer 118 and a reflective layer 120. The light guide 126 transfers the light emitted from the light emitting layer 118 to the light-recycling envelope 112 through an opening 127 in the envelope 112. Part of the source light gets absorbed by wavelength conversion layer 124 and emitted at a second wavelength band. Recycling of the source light within the envelope 112 helps convert more of it into the second wavelength band. Some of the source light and converted light leave the envelope 112 through the opening 127 and get guided by the light guide 126 back toward the LED 116. The reflective layer 120 of LED 116 reflects part of the source light and converted light toward the envelope 112. Some of the light exiting through the output aperture 114 gets transmitted and the remainder gets reflected back toward the envelope 112 by optical element 125. This process continues until all the light within the envelope 112 is either transmitted through optical element 125, absorbed or lost. Illumination system 200 delivers light with enhanced brightness when compared to the brightness of the source and converted light beams. However, illumination system 200 is not efficient in light recycling due to the limited reflectivity of the reflective layer applied to the interior surface of light-recycling envelope 112. Therefore, systems with enhanced recycling efficiency are required in order to reduce light losses and improve the overall efficiency.
In U.S. Pat. No. 7,070,300, Harbers et al. proposes illumination system 300 having a wavelength conversion element 212 that is physically separated from the light source 202 as shown in FIG. 1C. Illumination system 300 consists of a wavelength conversion element 212 (e.g. phosphor), a light source 202 (e.g. LED) mounted over an optional submount 204, which is in turn mounted on a heatsink 206, a first light collimator 208 to collimate light emitted from the light source, a color separation element 210, a second light collimator 214 to collimate light emitted from the wavelength conversion element 212, a first radiance enhancement structure 222 (e.g. a dichroic mirror or a diffractive optical element) mounted over the wavelength conversion element 212, a highly reflective substrate 215 mounted over a heatsink 216, a second radiance enhancement structure 218 (e.g. diffractive optical element, micro-refractive element, or brightness enhancement film) and a polarization recovery component 220. Light emitted from light source 202 is collimated by first light collimator 208 and directed toward the second light collimator 214 by color separation element 210. Second light collimator 214 concentrates a certain amount of this light on the wavelength conversion element 212, which in turn converts part of the source light into a light having a second wavelength band (i.e. converted light). This converted light gets collimated by the second light collimator 214 and transmitted by the color separation element 210 toward the second radiance enhancement structure 218, which in turn passes part of this light toward the polarization recovery component 220 and reflects the remainder toward the wavelength conversion element 212. The polarization recovery component 220 passes light with one polarization state and reflects the other state toward wavelength conversion element 212.
In U.S. Pat. No. 7,234,820, Harbers et al. proposes illumination system 400 having light collimators 375 and 381 having reflective apertures 390 and 391 for the purpose of enhancing the brightness of delivered light. As shown in FIG. 1D, illumination system 400 is comprised of a wavelength conversion element 374 (e.g. phosphor) mounted on a heatsink 376, a first fan 377, a light source 376 (e.g. LED) mounted on a heatsink 386, a second fan 387, a first light collimator 375 to collimate converted light emitted from the wavelength conversion element 374, a first reflective aperture 390 at the exit face of the first light collimator 375, a dichroic mirror 382, a second light collimator 381 to collimate light emitted from the light source 376, a second reflective aperture 391 at the exit face of second light collimator 381, and light tunnel 384. Light emitted from light source 376 is collimated by first light collimator 381 and directed toward the second light collimator 375. Some of this light exits the second reflective aperture 391 and the remainder gets reflected back toward the light source 376. The second light collimator 375 concentrates the light received through its reflective aperture 390 on the wavelength conversion element 374, which in turn converts part of the source light into a light having a second wavelength band (i.e. converted light). This converted light gets collimated by the first light collimator 375 and part of it passes through the first reflective aperture 390 toward the dichroic mirror 382, which in turn reflects the converted light toward light tunnel 384.
Illumination systems 300 and 400 are not compact. In addition, these systems 300 and 400 are not efficient in light recycling due to the limited reflectivity of the reflective layers utilized in these systems 300 and 400, especially, the reflective coatings that are located directly below the wavelength conversion element 212 and 374. Therefore, systems with more compactness and enhanced recycling efficiency are needed in order to reduce light losses and improve the overall optical and electrical efficiencies.
Known wavelength conversion based illumination systems suffer from limited efficiency, limited compactness and lack of control over spatial distribution of light delivered in terms of intensity and angle. Therefore, there is a need for compact, light weight, efficient and cost-effective illumination systems that provide control over spatial distribution of light in terms of intensity and angle over a certain area such as the active area of a display panel. Such illumination systems enable miniature projection systems with smaller light valves (˜0.2″) leading to more compactness and less expensive projection systems.